The measurement of electric circuit parameters, such as resistance and conductance using ratiometric techniques, is well known. Many ratiometric measurement circuits use a dual slope analog-to-digital(A/D) converter to measure the unknown value of a parameter. These A/D converters are commonly referred to as ratiometric converters when used in ratiometric measurement circuits. Ratiometric converters consume relatively small amounts of power and, hence, have found widespread use in battery powered devices, such as portable measurement instruments.
Many times the circuit parameters to be measured are located in "noisy" environments, such as telephone lines, for example, in which the noise exists as leakage current and/or a.c. interference. As is well known, noise may reduce the accuracy of measurement instruments. In fact, the accuracy of a ratiometric measurement instrument depends, at least in part, on the noise rejection capability of the ratiometric measurement circuit. That is, to some degree, the better the signal-to-noise ratio of the ratiometric measurement circuit, the higher the accuracy of the measurement instrument.
One way that prior art ratiometric measurement circuits achieve improved noise rejection is by operating with relatively high voltages, such as power supply voltages of .+-.15 volts and a reference voltage of 10 volts, for example, as opposed to lower power supply and references voltages, such as .+-.3 volts and 1 volt, respectively. The use of a higher reference voltage results in an increased signal-to-noise ratio and, hence, improved noise rejection by the prior art measurement circuits. However, the higher reference voltage requires using ratiometric converters with greater dynamic range. Unfortunately, the increased dynamic range translates into increased power consumption by the ratiometric converter, which is highly undesirable for battery powered circuits, such as those found in portable measurement instruments.
Accordingly, there is a need for a ratiometric measurement circuit that has improved noise rejection and achieves this improved noise rejection without consuming a significant amount of additional power. Further, the improved noise rejection should require a minimal number of additional components so that the entire measurement circuit may be easily packaged in a portable instrument. The present invention is directed to a ratiometric measurement circuit designed to achieve these results.